Ten mixed-breed bitches of unknown ages, weighing 10 to 15 kg, were given a series of subcutaneous injections of follicle-stimulating hormone according to 2 different schedules, for total doses of 35 or 50 mg. Forty-eight hours after the last injection of follicle-stimulating hormone, each bitch was given 15 mg of luteinizing hormone, intravenously. The internal genitalia of 8 of the 10 bitches were recovered 24 hours after the luteinizing hormone injection and were examined histologically. External signs of estrus developed in 7 of 10 bitches in the 2 groups, indicating that estrogen was being secreted by the ovaries. Histologic examination of the ovaries revealed that many follicles had grown to the antral stage, but none had developed to the preovulatory stage, and all were atretic. Ovulation did not occur in any bitch. Endometrial proliferation and vaginal cornification that occurred in 7 of the 10 bitches were further evidence of estrogen production by the ovaries.